Gate technology for strained surface channel and strained buried channel MOSFET devices

ABSTRACT

A method of fabricating a semiconductor device including providing a semiconductor heterostructure, the heterostructure having a relaxed Si 1-x Ge x  layer on a substrate, a strained channel layer on the relaxed Si 1-x Ge x  layer, and a Si 1-y Ge y  layer; removing the Si 1-y Ge y  layer; and providing a dielectric layer. The dielectric layer includes a gate dielectric of a MISFET. In alternative embodiments, the heterostructure includes a SiGe spacer layer and a Si layer.

PRIORITY INFORMATION

This application claims priority from provisional application Ser. No.60/223,595 filed Aug. 7, 2000.

BACKGROUND OF THE INVENTION

The invention relates to gate technology for strained surface channeland strained buried channel MOSFET devices.

The advent of high quality relaxed SiGe layers on Si has resulted in thedemonstration of field effect transistors (FETs) with carrier channelsenhanced via strain. The strain can be incorporated in the channel dueto the lattice mismatch between the channel and the relaxed SiGe createdby a change in the Ge concentration between the channel layer and therelaxed SiGe layer. For example, a Ge concentration of 20% Ge in therelaxed buffer is high enough such that a thin strained Si layer canexhibit electron mobilities as high as 1000-2900 cm²/V-sec. Also, if theGe concentration in the channel is greater than the concentration in thebuffer, hole channel mobilities can be enhanced. For example, a relaxedbuffer concentration of 60-70% Ge can compressively strain a Ge channellayer, creating potentially extremely high hole mobilities.

Although the exact physics of carrier scattering are not known insideshort-channel FETs, one thing is clear: these enhanced mobilitiestranslate into increased device performance, even at very short gatelengths. In addition to higher speed and a different power-delayproduct, the use of strained channels allows for the incorporation ofnew FET structures into Si-based circuits. Thus, it is anticipated thatthe high performance, new flexibility in device design, and economics ofusing a Si-based platform will lead to a plethora of new circuits andproducts.

With regards to these new circuits and products, the devices based onmetal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS)gate technology are the most intriguing, since these devices can followvery closely the processes already used in Si VLSI manufacturing. Twomain types of devices are of particular interest: the surface channeldevice and the buried channel device, examples of which are shown inFIGS. 1A and 1B.

FIG. 1A is a cross section of a block diagram of a strained Si surfacechannel device 100, in which a thin strained Si layer 102 is grown atopa relaxed SiGe virtual substrate. The SiGe virtual substrate can berelaxed SiGe 104 on a SiGe graded buffer 105 (as shown in FIG. 1 a),relaxed SiGe directly on a Si substrate 106, or relaxed SiGe on aninsulator such as SiO₂. The device also includes a SiO₂ layer 108 andgate material 110.

FIG. 1B is a cross section of a block diagram of a strained Si buriedchannel device 112, in which a SiGe layer 116 and a second strained Silayer 120 (used for gate oxidation) cap the strained Si channel layer114. The structure also includes a graded SiGe buffer layer 125 and asecond relaxed SiGe layer 126. In both device configurations, a gateoxide 122 is grown or deposited and the gate material 124 is depositedto form the (MOS) structure. Although only devices with strained Sichannels are shown in FIGS. 1A and 1B, the invention is applicable toany heterostructure device fabricated on a relaxed SiGe platform. Forexample, the heterostructure strained channel could be Ge or SiGe of adifferent Ge content from that of the underlying SiGe virtual substrate.However, the following description will focus on the applicability ofthe invention to the strained Si device variants illustrated in FIGS. 1Aand 1B.

In order to form the MOS gate of the heterostructure device, the SiGewould ideally be oxidized directly in the buried channel device, and thestrained Si would be oxidized directly in the surface channel device.Unfortunately, there are problems due to the nature of the Si/SiGeheterostructures in both cases that render the direct oxidation processunsatisfactory.

First consider the surface channel device. Since Si is being oxidized,the interface state density at the resulting SiO₂/Si interface is low,and an electrically high quality interface results. However, alloxidation and cleaning processes during the device and circuitfabrication consume the Si material. In conventional Si processing,there is generally little worry about Si consumption since so littlematerial is consumed compared to any limiting vertical dimension earlyin the fabrication process. However, in the case of the strained surfacechannel FET described here, the top strained Si layer is typically lessthan 300 Å thick, and thus too much Si consumption during cleaning andoxidation steps will eliminate the high mobility channel.

One obvious solution is to simply deposit extra Si at the surface,planning for the removal of the Si that occurs during processing.However, the channel strain, which gives the channel its higher carriermobility, limits the Si layer thickness. At a great enough thickness,the Si layer will begin to relax, introducing misfit dislocations at theSi/SiGe interface. This process of dislocation introduction has twodeleterious effects on device performance. First, the strain in the Siis partially or completely relieved, potentially decreasing the carriermobility enhancements. Second, dislocations can scatter carriers,decreasing carrier mobility. Dislocations can also affect device yield,reliability, and performance.

The buried channel case appears to be a better situation at first, sincethe Si layer thickness is buried. However, in this case, directoxidation of SiGe creates a very high interface state density at theoxide/SiGe interface, leading to poor device performance. A knownsolution in the field is to create a thin Si layer at the surface of theburied channel structure. In this structure, the surface layer iscarefully oxidized to nearly consume the entire top Si layer. However, athin layer of un-oxidized Si is left so that the interface to the oxideis the superior SiO₂/Si interface rather than the problematic oxide/SiGeinterface. Although this sacrificial surface Si layer solves theinterface electronic property issue, the structure now has the samelimits as the structure described above, i.e., the sacrificial Si layerwill be slowly etched away during Si processing, possibly leading toexposure of the SiGe and degradation of the electrical properties of theinterface as described.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a method offabricating a semiconductor device including providing a semiconductorheterostructure, the heterostructure having a relaxed Si_(1-x)Ge_(x)layer on a substrate, a strained channel layer on the relaxedSi_(1-x)Ge_(x) layer, and a Si_(1-y)Ge_(y) layer; removing theSi_(1-y)Ge_(y) layer; and providing a dielectric layer. The dielectriclayer includes a gate dielectric of a MISFET. In alternativeembodiments, the heterostructure includes a SiGe spacer layer and a Silayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross sections of block diagrams of strained Sisurface and buried channel devices, respectively;

FIGS. 2A and 2B are cross sections of block diagrams of startingheterostructures for surface channel and buried channel strained MOS,respectively, in accordance with the invention;

FIGS. 3A-3D are block diagrams showing the process sequence for astrained surface channel MOS device;

FIGS. 4A-4D are block diagrams showing the process sequence utilizingthe gate structure for a buried channel device;

FIG. 5 is a graph of oxidation rates, under a wet oxidation ambient at700° C., of SiGe alloys, with Ge contents of 0.28 and 0.36, compared tothe oxidation rate of bulk silicon;

FIG. 6 is a graph showing the oxide thickness of both a Si_(0.7)Ge_(0.3)alloy and a Si/Si_(0.7)Ge_(0.3) heterostructure;

FIG. 7 is a cross-sectional transmission electron micrograph (XTEM) ofthe Si/Si_(0.7)Ge_(0.3) heterostructure;

FIG. 8 is a XTEM image of the identical Si/Si_(0.7)Ge_(0.3)heterostructure after wet oxidation followed by oxide removal via a wetetch;

FIG. 9 is a structure for a buried channel MOSFET using relaxed SiGe andstrained Si in accordance with the invention; and

FIG. 10 is a graph showing a plot of the middle SiGe layer thickness(h2) and the resulting misfit dislocation spacing.

DETAILED DESCRIPTION OF THE INVENTION

To eliminate the issue of losing valuable surface Si, an innovative stepthat has not been previously considered can be employed. In fact, anyinterest in this area is dominated by discussions of how to change theSi device and circuit process to conserve Si consumption. Although theseare certainly possibilities, such constraints severely limit processflexibility, alter the process further from the conventional Si process,and most likely will increase the cost of the fabrication process.

A solution for the buried channel and surface channel structures is toactually deposit another SiGe layer after the desired device structure(which, in the buried channel heterostructure, includes the sacrificialSi layer for oxidation). The structures are shown in FIGS. 2A and 2B.

FIG. 2A is a cross section of a block diagram of a startingheterostructure 200 for surface channel strained MOS in accordance withthe invention. The structure 200 includes a Si substrate 202, a SiGegraded buffer 204, a relaxed SiGe layer 206, and a strained-Si channellayer 208. FIG. 2B is a cross section of a block diagram of a startingheterostructure 214 for buried channel strained Si MOS. The structure214 includes a Si substrate 216, a SiGe graded buffer 218, relaxed SiGelayers 220 and 230, a first strained-Si channel layer 222 and a secondstrained-Si layer 224 for the gate oxide.

These structures are identical to those depicted in FIGS. 1A and 1Bbefore the gate stack formation, except for the addition of a SiGecapping layer 210, 226 and an optional Si capping layer 212, 228. Sincethe SiGe layer 210, 226 is closely lattice-matched to the relaxed SiGelayers below the device layers, there is essentially no limit on thethickness of the SiGe layer. This SiGe layer thickness can be tuned tothe thickness of material removed before gate oxidation, so that thestrained Si layer is exposed just before oxidation. Alternatively, theSiGe can be thicker than the removal thickness and then can beselectively removed. In fact, as described below, SiGe can beselectively removed with respect to Si using a variety of conventionalSi-based processes. Therefore, cleaning and oxidation steps can beperformed during the Si device and circuit fabrication process withlittle worry of consuming the precious strained Si and/or thesacrificial strained Si. One only needs to create a SiGe thick enoughsuch that it is not totally consumed before the critical gate oxidationstep.

An additional option can be to place yet another Si layer 212, 228 ontop of the additional SiGe layer 210, 226. In some processingfacilities, the idea of SiGe on the surface, instead of Si, is a factorfor concern. In this case, another Si layer can be deposited on top ofthe additional SiGe layer described above. By choosing the Geconcentration in the additional SiGe layer to be greater than that ofthe virtual buffer, a compressive layer can be created; thus, if thisadditional optional Si layer is greater than the critical thickness,there is no possibility of dislocations moving into the device layers.This phenomenon occurs since the Si layers are tensile, and thereforedislocations introduced into the top optional Si layer have a Burgersvector that will not allow them to glide favorably in the compressivelayer below. The dislocations in the top optional Si layer (if the Silayer critical thickness is exceeded) will not penetrate into the layersbeneath it, and therefore as much Si can be deposited as desired. Infact, this optional Si capping layer need not be strained at all in thiscase and can serve as a protective sacrificial layer even if it is fullyrelaxed.

FIGS. 3A-3D are block diagrams showing the process sequence for astrained surface channel MOS device utilizing the gate structuredescribed above (the process is shown for a structure without anoptional strained surface layer). FIG. 3A shows the initial Si/SiGeheterostructure 200 shown in FIG. 2A. FIG. 3B shows the structure afterthe completion of the initial steps of a Si VLSI process, which couldinclude wet chemical cleans and oxidation steps. Thus, in FIG. 3B, theprotective SiGe capping layer 210 has been reduced in thickness, as aportion of the layer has been consumed during processing. Next, theremainder of the protective SiGe capping layer 210 is selectivelyremoved, leaving the underlying Si layer 208 intact and exposed. Asacrificial oxidation step and oxide strip can also be performed at thispoint to improve the quality of the exposed Si surface.

The resulting structure is shown in FIG. 3C. FIG. 3D shows the finaldevice structure after gate oxidation to form a gate oxide 300, astructure in which the minimum possible amount of Si was consumed priorto the gate oxidation step. Alternatively, at this point an alternategate dielectric could be deposited on the exposed Si surface. A pristineSi surface is just as important for a high quality interface with manydeposited gate dielectrics as it is for a thermally grown SiO₂ gatedielectric.

FIGS. 4A-4D are block diagrams showing the process sequence utilizingthe gate structure for a buried channel device (the process is shown fora structure without an optional strained surface layer) using theinitial Si/SiGe heterostructure 214 shown in FIG. 2B. The process stepsare identical to those of FIGS. 3A-3D, but in the final heterostructure,the Si channel layer 222 is separated from the gate dielectric 400 by aSiGe spacer layer 220, thus forming a buried channel. Using selectiveprocesses to etch down to the buried Si channel or the top Si layer canuse the starting heterostructure 214 in FIG. 4A to form a surfacechannel device. Such a process can result in enhancement mode anddepletion mode devices that can in turn be used to create E/D logiccircuits as well as a plethora of analog circuits.

In both sequences, an exemplary sequence of steps is: 1.Pre-gate-oxidation cleaning steps and oxidation; 2. Selective etch oroxidation to remove residual protective SiGe layer; 3. Sacrificial oxideformation on Si; 4. Sacrificial oxide strip: 5. Gate oxidation.

It will be appreciated that steps 3 and 4 can be optional, depending onwhether there may be a small amount of Ge left on the surface after theselective removal of the SiGe protection layer. When the originalheterostructure is grown, the SiGe/Si interface will not be infinitelyabrupt, and therefore it is possible to have a small amount of Ge in theoptimally pure Si layer. A sacrificial oxide step can be employed toremove an additional small amount of the Si layer to ensure that pure Siis oxidized in the gate oxidation step, ensuring high quality gateoxide.

The second step, the selective removal of the residual SiGe protectivematerial, can be accomplished in a variety of ways. One convenientprocess is a wet oxidation step, preferably at 750° C. or below. Underwet oxidation at these temperatures, SiGe is oxidized at rates that canbe 100 times greater than rates oxidizing Si under the same conditions.Thus, in order to expose the Si for gate oxidation, one can simply do awet oxidation of the SiGe layer and selectively stop at the Si layer.The oxidized SiGe can be stripped to expose the Si. It is important tonote here that the low temperature is not only important for theselectivity in the oxidation process, but also the low temperature isimportant to minimize or prevent the snow-plowing of Ge in front of theoxidation front, a known problem in the direct oxidation of SiGe.

FIG. 5 is a graph of oxidation rates, under a vet oxidation ambient at700° C., of SiGe alloys, with Ge contents of 0.28 and 0.36, compared tothe oxidation rate of bulk silicon. It is evident from the graph that,under such conditions, the oxidation rate of SiGe increases as the Gecontent of the film increases.

FIG. 6 is a similar graph, showing the oxide thickness of both aSi_(0.7)Ge_(0.3) alloy and a Si/Si_(0.7)Ge_(0.3) heterostructure. Again,the oxidation conditions were 700° C. in a wet ambient; however, FIG. 6depicts very short oxidation durations compared to FIG. 5. TheSi/Si_(0.7)Ge_(0.3) heterostructure consists of a 50 Å strained Siburied layer, followed by a 30 Å Si_(0.7)Ge_(0.3), a 20 Å strained Silayer and finally a 50 Å Si_(0.7)Ge_(0.3) capping layer.

A cross-sectional transmission electron micrograph (XTEM) of theSi/Si_(0.7)Ge_(0.3) heterostructure is shown in FIG. 7. It should benoted from FIG. 6 that the presence of strained Si layers in theheterostructure results in a dramatic retardation in the oxidation ratewhen compared to the oxidation rate of the uniform Si_(0.7)Ge_(0.3).This retardation of the oxidation rate forms the basis of the selectiveremoval of SiGe alloys over strained Si epitaxial layers.

FIG. 8 is a XTEM image of the identical Si/Si_(0.7)Ge_(0.3)heterostructure after wet oxidation at 700° C. for 2 minutes followed byoxide removal via a wet etch. It is apparent that the thin strained Silayer is unaffected by the selective oxidation and remains fully intact.Based on the data shown in FIG. 5, an oxidation duration of 2 minutesfar exceeds that required to fully oxidize the 50 Å Si_(0.7)Ge_(0.3)capping layer of the heterostructure. The very thin dark band, which isapparent on the surface of the strained Si layer, is a snow-plowed highGe content layer that occurs during oxidation. Such a layer may beremoved using a simple chemical clean or a sacrificial oxidation step,either or both of which typically occur prior to the formation of thegate oxide.

Alternatively, the protective SiGe capping layer can be removed viaselective dry or wet chemical etching techniques. For example, at highpressures (>200 mT) and low powers, CF₄ dry etch chemistries wilt etchrelaxed SiGe films with high selectivity to Si. Mixtures of hydrofluoricacid (HF), hydrogen peroxide (H₂O₂), and acetic acid (CH₃COOH) will alsoselectively etch relaxed SiGe layers over Si at selectivities of 300:1or more. Other potential selective wet chemical mixtures include HF,water (H₂O), and either H₂O₂ or nitric acid (HNO₃).

Additionally, the stability of the entire structure can be improved byincreasing the Ge concentration in the intermediate SiGe layer, and alsothe top SiGe layer if desired. Below, energetic calculations are used toreveal a guide to creating semiconductor layer structures that increasestability with respect to misfit dislocation introduction.

The critical thickness for a buried channel MOSFET using relaxed SiGeand strained Si has been determined using the energy-balanceformulation. The structure considered is the one shown in FIG. 9. Thestructure 900 includes a 30% SiGe virtual substrate 902 topped by a 80 Åstrained Si layer 904, a SiGe layer with Ge concentration x2 andthickness h2 906, and an additional 30 Å of strained Si 908. Additionalstability would result from the addition of an additional SiGe cap layeras described previously. To simplify, the example of FIG. 9 considersonly the increased stability created by increasing the Ge concentration(x2) or thickness (h2) of the SiGe intermediate layer. Additionally,since the SiGe cap layer is removed during processing, the stability ofthe heterostructure with the SiGe cap removed is or primary importance.

In device processing, one must consider the critical thickness of theentire structure with respect to the relaxed virtual substrate.Individual layers that exceed the individual critical thicknesses arenot explicitly ruled out, so one practicing the art would have to verifythat none of the layers that are introduced into the desired structureexceed the individual layer critical thicknesses. In other words, in thefollowing calculation it is assumed that each layer in the structure isbelow its critical thickness with respect to the relaxed buffer.

One key to the formulation is to realize that this calculation should bedone with respect to the plastic deformation of the layer composite, δ.Then, the dislocation array energy is the same expression regardless ofthe layer structure. The elastic energy in the individual layers ischanged because of δ. In tensile layers, the strain is lowered by δ. Incompressive layers, the energy is raised by δ.

Thus, the energy for a dislocation array (per unit area) inserted at thebase of the composite is:E _(δ)=2δD(1−ν cos α)[ln(h _(T) /b)+1]where h_(T) is the total thickness of the composite (h₁+h₂+h₃), α is theangle between the dislocation line and the Burgers vector b, ν is thePoisson ratio, and D is the average shear modulus for a dislocationlying at the interface between the virtual substrate and the compositestructure.

The total elastic energy (per unit area) in all the layers is:$E_{t} = {\sum\limits_{i = 1}^{3}{{Yɛ}_{i}^{2}h_{i}}}$where Y is the Young's modulus. Thus, the total energy of the system is:E _(T) =E _(δ)+E_(ε).

The energy can now be minimized with respect to δ (if the energy islowest with no dislocations, then δ will have a less than or equal tozero value). The value of plastic deformation then is (for the 3 layerexample):$\delta = {\frac{f_{1}h_{1}}{h_{T}} - \frac{f_{2}h_{2}}{h_{T}} + \frac{f_{3}h_{3}}{h_{T}} - \frac{{D\left( {1 - {\upsilon\quad{\cos\quad}^{2}\alpha}} \right)}\left\lbrack {{\ln\left( \frac{h_{T}}{b} \right)} + 1} \right\rbrack}{{Yh}_{T}}}$

The examination of this solution reveals that a general formulation forany structure would be (for any structure of n layers):$\delta = {{\sum\limits_{i}^{n}\frac{f_{i}h_{i}}{h_{T}}} - \frac{{D\left( {1 - {\upsilon\quad{\cos\quad}^{2}\alpha}} \right)}\left\lbrack {{\ln\left( \frac{h_{T}}{b} \right)} + 1} \right\rbrack}{{Yh}_{T}}}$where f has been assigned a negative value for compressive layers andpositive value for tensile layers, and h_(T) is the total thickness ofthe structure: $h_{T} = {\sum\limits_{i}^{n}{h_{i}.}}$

The amount of plastic deformation and resulting misfit dislocationspacing S was calculated for the structure depicted in FIG. 9 asfollows:

-   -   Lower strained Si layer thickness h1=80 Å    -   Upper strained Si layer thickness h3=30 Å    -   Middle SiGe layer thickness h2 variable    -   Middle SiGe layer Ge concentration x2 variable    -   Virtual substrate GC concentration: 30%

FIG. 10 is a graph showing a plot of the middle SiGe layer thickness(h2) and the resulting misfit dislocation spacing. The sharp upturn onthe plots represents the critical thickness h2 of the middle SiGe layerwhen the entire composite structure destabilizes and introducesdislocations at the channel/virtual buffer interface. The differentcurves are for the different compositions in the second layer h2. Verysmall increases in Ge result in a large jump in stability of the devicelayers. This suggests that it is possible to stabilize the layersignificantly but not have the band structure altered that much. Addingan extra 5-10% Ge into the h2 layer increases the stability drastically.For example, FIG. 10 indicates that over 100 Å of 30% Ge is required toprovide the stability of a 20 Å layer of 45% Ge content.

Increasing h2 even when the h2 layer is lattice-matched to the virtualbuffer increases the stability of the multilayer structure. In theequations above, the effect can be seen to be much weaker than when acompressive strain in h2 is created. When f2 is zero due to latticematching to the virtual buffer, the increased stability with increasingh2 comes from the fact that h_(t) is increasing and therefore decreasingδ (and increasing S).

It will be appreciated that all the calculations are equilibriumcalculations, and as usual, one might suspect that these numbers aresomewhat conservative, although also consider that the layers possessmany threading dislocations that can bend over at the criticalthickness, so there are plenty of sources for misfit dislocationgeneration.

Sacrificial SiGe capping layers provide an innovative method for theprotection of thin strained device layers during processing. Such layersshield these critically important strained channel layers from processsteps, such as wet chemical cleans and oxidations, which consume surfacematerial. Before the growth or deposition of the gate dielectric, theseprotective SiGe layers can be selectively removed by standard processessuch as oxidation or wet etching, revealing the intact strained devicelayer. Also presented is a guideline for engineering strained layerstacks such that relaxation via misfit dislocation is prevented.Compressively strained intermediate layers increase the stability oftensile channel layers, and also serve as a barrier for misfitdislocation introduction into the underlying layers.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1-39. (canceled)
 40. A method of fabricating a semiconductor devise, themethod comprising the steps of: (a) providing a semiconductorheterostructure, the heterostructure comprising a substrate and astrained layer disposed thereover; (b) providing at least onesacrificial layer disposed over the strained layer; (c) removing atleast a portion of the at least one sacrificial layer, wherein thestrained layer remains intact after removal; and (d) providing a gatedielectric over the strained layer.
 41. The method of claim 40 whereinthe strained layer comprises a semiconductor material selected from thegroup consisting of Si, Ge, and SiGe.
 42. The method of claim 40 whereinthe strained layer has thickness of less than about 300 angstroms. 43.The method of claim 40 wherein the strained layer is substantially freeof misfit dislocations.
 44. The method of claim 40 wherein the at leastone sacrificial layer comprises Si or SiGe.
 45. The method of claim 40wherein step (b) comprises providing a plurality of sacrificial layersdisposed over the strained layer, the plurality of sacrificial layersincluding at least one sacrificial layer comprising Si and at least onesacrificial layer comprising SiGe.
 46. The method of claim 40 whereinthe substrate comprises Si.
 47. The method of claim 46 wherein thesemiconductor heterostructure comprises a relaxed Si_(1-x)Ge_(x) layerdisposed between the substrate and the strained layer.
 48. The method ofclaim 47 wherein the semiconductor heterostructure comprises a bufferlayer disposed between the substrate and relaxed Si_(1-x)Ge_(x) layer.49. The method of claim 46 wherein the semiconductor heterostructurecomprises an insulator layer disposed over the substrate, the strainedlayer being disposed over the insulator layer.
 50. The method of claim40 wherein at least a portion of the at least one sacrificial layer isremoved using a technique selected from the group consisting of: wetoxidation, dry oxidation, wet etching, and dry etching.
 51. The methodof claim 50 wherein at least a portion of the at least one sacrificiallayer is removed using the wet oxidation technique at a temperature upto about 750° C.
 52. The method of claim 40 wherein step (d) comprisesoxidizing at least a portion of the strained layer thereby providing thegate dielectric thereon.
 53. The method of claim 40 wherein step (d)comprises depositing a dielectric layer over at least portion of thestrained layer thereby providing the gate dielectric.
 54. A method offabricating a semiconductor device, the method comprising the steps of:(a) providing a semiconductor heterostructure, the heterostructurecomprising a substrate and a strained layer having first thicknessdisposed thereover; (b) providing at least one sacrificial layerdisposed over the strained layer; (c) removing the at least onesacrificial layer and a portion of the strained layer, the portionhaving second thickness substantially less than the first thickness; and(d) providing a gate dielectric over the strained layer.
 55. The methodof claim 54 wherein the strained layer comprises a semiconductormaterial selected from the group consisting of Si, Ge, or SiGe.
 56. Themethod of claim 54 wherein the first thickness is less than about 300angstroms.
 57. The method of claim 54 wherein the substrate comprisesSi.
 58. The method of claim 57 wherein the semiconductor heterostructurecomprises a relaxed Si_(1-x)Ge_(x) layer disposed between the substrateand the strained layer.
 59. The method of claim 58 wherein thesemiconductor heterostructure comprises a buffer layer disposed betweenthe substrate and relaxed Si_(1-x)Ge_(x) layer.
 60. The method of claim57 wherein the semiconductor heterostructure comprises an insulatorlayer disposed over the substrate, the strained layer being disposedover the insulator layer.
 61. The method of claim 54 wherein at leastone the at least one sacrificial layer and a portion of the strainedlayer are removed using a technique selected from the group consistingof: wet oxidation, dry oxidation, wet etching, and dry etching.
 62. Themethod of claim 54 wherein at least one the at least one sacrificiallayer and a portion of the strained layer are using the wet oxidationtechnique at a temperature up to about 750° C.
 63. The method of claim54 wherein step (d) comprises oxidizing at least a portion of thestrained layer thereby providing the gate dielectric thereon.
 64. Themethod of claim 54 wherein step (d) comprises depositing a dielectriclayer over at least portion of the strained layer thereby providing thegate dielectric.
 65. The method of claim 54 wherein the strained layeris substantially free of misfit dislocations.
 66. The method of claim 54wherein the second first thickness is less than about 10 angstroms.